Crack Growth in GR-S Tread Stocks RELATION TO STATE OF CURE AND CO_MPOSITIOA' .A. E. JPVE The B . F. Goodrich C o m p a n y , Akron, Ohio 1 ' w o of the most serious troubles enwunterecl Jtith GK-Y in tire use are ewessibel: rapid cracl. growth and high teniperature build-up in the tread coniposition. One characteristic m a y be improbed a t the expense of the other by
tariations which affect the state of cure. The relations between these properties, when measured under a specific set of conditions, \t ere studied atid empirical formulas deieloped which arc \ d i d for ii wide bariet? of co~iipounding modification-. iL n o Ieclpe ~ of theie relation8 pcrniitthe rend? ebaluation of experiniental pol?niers or suggested conipounding modifications \t hic.11 are intended t o probide a niore faborable balance betm eeii these propertie?. .in inipro\ement in this balance ma? be eupreised iis a ratio which for con\eniencc has been termed a qualit) index.
A \vide varic.ty oi GR-S trc:id e ~ i n i p o ~ i t ion d ~ ~,vliichcrack k r o \ v t I ~:UK[ t c n i p x t u r e i i w d:ita liati i.)een obtaiiie(i were first qtuditd l)y plutting t lic values f i ~ rtemperat urc risis lit'-po of c.oiiipound sliould ti(! much highi.1 t hail the ralculiited values. '111 of the‘ data used in these dcrivations and illustration. \\-('le obtained 011 a singlc D e I I a t t i a type Hexing machine. It \vas of t t o rlcrcmiine ~ r h r t h e ra similar i.c>lntionapplied t o otlic.1, ni:irhinv~, -I hc,rics I)! six tread compounds \\-itli varying propo1,tiotis of wriiig agents was tested on tn-o iitliisr nischines. The first, kiiun-11 as the horizontal Dr IIattia. i.. qimi1;ir to the one u s d ax the stantlard except that it is opi,ixtetl at 500 cyclei per niiiiute with a atyolie of 2 inches. The type niachine not previously tlesci,ibcd in the literature, in nhich nioltic~tlsamplcs of .,-inch-iquare crois scc:ion, having the same e groove :is t h e stantlard De Matria sample, are fastol a l r l t run ov('r smnll diameter pulleys a t a rate which tc~tii~ girc.9 the sample 400 Hvxurc'; pcsr minute. The results arc rep o i ~ t c das the numtx,r of cyclw t o ~ ~ I J ~ U C aC .crack SO', of the total ividth of the saniplt.. In Iioth t c y r b tnclve samples of c,acli stock n e r e tcstccl on each n~achine. Thc result are illustrated i n 1:igurc. 2. Tlic, hctlt-typcl machine .evere as the horizontal high speed De I l a t t i a , ant1 both ai'e con4clerably more severe than the vertical The .lopes of the curves are approximat ply the sanl tliv cui'vc shown in Figure 1, n.liicii iiitlictites That n-ill apply t o a variety of Hcxiiig niachiiit',: with a suitahk rrvi\ion of the constant. Thcb rate of crack g r o v r h in GIt-3 trcacl ~ i J l i l ~ ~ l ~ Uiiioi~ liti.~ rapidly with a n increase in t e s t iiig t i ~ m j i c ~ r a t u r TIJ ~ ~ . tlcit~riiiiiie lion- t h e curve relating c.rac.1~ growth t o hysteresis tcmpcrature riw is shifted t)y an inin testing tenipcraturv, is conipoundl usc'd in this comparison n-ere twteti on tlic'belt-type i~esingriiac.liinc~ at 158' and a t 212' F. FigU I T 3 shon-s the results ohtaiiiccl at these temperatur w l l :is at room teniperatui~c~. (hi'of the primary reasom flJr the study of t!iis rclntioii txlt \ v w n crncli gron-th an[l tt~nipc:rature rise \vas for thc, pui'pose of evaluating supposctl improvements in the CiR-8 type of copolymcr n-ith
,
1497
of GI
